Vessels for collecting material from an underwater bottom are known.
Examples of such vessels are dredging vessels, such as trailing suction hopper dredgers, cutter suction dredgers. Dredging vessels are used to maintain the depth of or deepen shipping channels, to dredge new shipping channels, and for sand and gravel extraction, for example for infrastructure projects and land reclamation.
Another example are a mining vessels, which are used to collect mining material from the underwater bottom. The mined material may comprise minerals, hydrates, diamonds, gold, copper, manganese, nickel, zinc.
The term collected material used in this text therefore refers to both dredged material and mined material. The collected material comprises a plurality of grains.
The collected material may be loaded into a loading space (hopper) or may be discharged via a discharge pipe line to a remote location.
On hopper dredgers the dredged material, also referred to as soil, is pumped into the loading space of the dredging vessel. In order to do this, the dredged material is mixed with water, creating a slurry, which can be pumped. When the slurry is in the loading space, the dredged material will settle.
The determination of the settling speed of the dredged material in the loading space of the dredging vessel is an important parameter for ensuring that the dredging is as efficient as possible. This also applies to mined materials which are stored in a loading space or processed by a concentration plant.
Depending on the type of work, widely differing materials are dredged or mined, which may have different typical sizes: sludge, clay, fine sand, coarse sand, gravel, rock, minerals, hydrates and often a combination thereof. The settling speed of the material depends on the grain size of the collected material.
The speed with which the loading space is filled and the ratio between the amount of water and dredged material in the slurry preferably matches the settling speed of the collected material to make the dredging or mining process as efficient as possible.
If the loading space is filled too quickly, a part of the collected material will not be given enough time to settle and will leave the ship once more via an overflow. Filling which is too slow is not attractive from a cost perspective. It is therefore very important to know how quickly the collected material settles.
To allow the loading process to run as efficiently as possible, it is necessary to know the settling speed: sludge does not settle, fine sand settles very slowly, coarse sand more quickly, whereas gravel and rock drop immediately. Depending on this settling speed and the associated speed at which the height of the settled bed rises, the dredging or mining process can be devised differently in terms of loading speed and density of the slurry, so that the settling process will run more efficiently.
Other types of operations, such as oil extraction operations, employ various methods for analysis of suspended fine particles in a fluid flow. For example, in WO2011/080216, a measuring unit is used that uses a camera with a light source for sampling images of a hydrocarbon fluid flow. The images are sent to an electronics unit for analyzing and measuring the very small suspended particles, such as oil droplet and solid particle concentrations separately to determine whether the fluid may cause damage due to excessive oil or solids droplet content. However, for mining or dredging operations, the particles are typically larger, meaning that they will settle and will affect flows during the process.
Different methods are known to optimize the dredging or mining process. For instance, the height of the settled bed can be monitored during dredging via radioactive means, or indirectly calculated by pressure measurements. Information about the determined height of the settled bed and the increase of the determined height can be used to control the dredging or mining process. Also, information about the average grain size of the collected material can be derived. One such way to derive this is disclosed in the article “Optimization of Sand and Gravel Extraction on a Dredger with Fully Automatic Online Particle Size Analysis.” For this, a sample is taken from the dredge, and particles from the sample are dried, put on a belt and then dropped off the belt. While the particles are free-falling, photos are taken and then analyzed.
However, such methods require relatively complicated and vulnerable sensor equipment and require separation and/or drying out the particles from the slurry. Also, such methods only provide information about an average grain size. Furthermore, as the increase of the settled bed is a relatively slow process and the other processes for analyzing require taking out a sample, drying and then analyzing; information about the average grain size is only available with a certain delay, typically in the order of 5-10 minutes. Consequently, adjusting the dredging or mining process can only be done with a delay.
In case the mixture of water and collected material is to be discharged via a discharge pipe line, for instance from a loading space or directly after it has been collected, detailed information about the grain size of the collected material is also advantageous, as this information can be used to determine an appropriate discharge flow speed, in which settling of the collected material inside the discharge pipe line is prevented or at least minimized. This aspect is important in both dredging and mining.